drop python<=3.7 support

According to https://endoflife.date/python python 3.7 has been
EOSed 27 Jun 2023.
Filter all code over `pyupgrade --py3.8-plus`.

Signed-off-by: Tomasz Kłoczko <[email protected]>

Author: kloczek

src/prompt_toolkit/layout/screen.py

@@ -169,7 +169,7 @@ def __init__(
 
         #: Escape sequences to be injected.
         self.zero_width_escapes: defaultdict[int, defaultdict[int, str]] = defaultdict(
-            lambda: defaultdict(lambda: "")
+            lambda: defaultdict(str)
         )
 
         #: Position of the cursor.

src/prompt_toolkit/layout/utils.py

@@ -5,7 +5,7 @@
 from prompt_toolkit.formatted_text.base import OneStyleAndTextTuple
 
 if TYPE_CHECKING:
-    from typing_extensions import SupportsIndex
+    from typing import SupportsIndex
 
 __all__ = [
     "explode_text_fragments",